Mercury vapour discharge lamp



Dec. 16, 1969 J. Jf vAN BOORT ETAL 3,484,637

MERCURY VAPOUR DISCHARGE LAMP Filed Feb. 26. 1968 INVENTOR 5 HENRICUS J.J. v. BOORT CORNELIS A.J. JACOBS United States Patent MERCURY VAPOUR DISCHARGE LAMP Henricus Johannes Joseph van Boort, and Cornelis Adrianus Joannes Jacobs, Emmasingel, Eindhoven, Netherlands, assignors, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Feb. 26, 1968, Ser. No. 708,277 Claims priority, application Netherlands, Mar. 1, 1967, 6703447 Int. Cl. H011" 1/02, 7/24 US. Cl. 313-25 4 Claims ABSTRACT OF THE DISCLOSURE A high-pressure mercury vapor discharge lamp containing an alkali iodide in which a conductor parallel to the discharge path and connected to one of the electrodes is surrounded by a tube of material impervious to ultraviolet radiation to prevent discoloration of the glass envelope as well as an increase in the ignition and operating voltages of the lamp.

It has been proposed to eliminate these disadvantages in that the conductor extending along the lamp is arranged at a maximum distance from the lamp and in that an outer envelope surrounding the discharge tube is shaped so that the two ends of the discharge tube are mechanically supported in this envelope. Because of the large distance between the conductor and the tube it is no longer able to support the discharge tube.

This has the disadvantage that at least the central part of the outer envelope must be wide. A further disadvantage is that it had appeared to be necessary to provide the outer envelope with a gas filling, for example, of nitrogen, which resulted in an undesirable increase in heat dissipation of the light source.

The invention provides a more satisfactory solution for avoiding these disadvantages and consists in that the conductor extending along the discharge tube is screened by an electrically non-conducting material which is substantially impervious to ultraviolet radiation.

For this purpose, use is preferably made of tubular glass screen, ceramic material and the like. The wall thickness of such a tube is preferably at least 0.2 mm.

The invention will be described with reference to a drawing, the sole figure of which shows an embodiment of the invention.

A straight high-pressure mercury vapor discharge tube is provided with electrodes 2 and 3 of which electrode 2 P CC is connected in the usual manner via a long lead-in and supporting wire 4 with the aid of a bipolar screw cap 6 to oneiend in the drawing end of the evacuated outer envelope 7 surrounding the assembly. A short supporting and leadin wire Sis provided for electrode 3.

In. the embodiment tube 1 is quartz glass and is provided with an ignition electrode 9 connected through a resistor 10 to the conductor 4. The distance between the electrodes 2 and 3 was approximately 50 mm. and the inner volume was approximately 15 cm. The tube 1 contained :60 mgs. of mercury which evaporated completely during normal operation and produced an operating voltage of approximately v. at a minimum wall temperature of approximately 580 C. (at a power of approximately 400 w.). The tube further contained approximately 3 mgs. of sodium iodide 10.3 mg. of thallium iodide and :0.06 mg. of indium iodide per cm.". The ignition gas used was argon at a pressure of approximately 20 mm.Hg.

According to the invention, the nickel wire 4 extending along the discharge tube 1 at a distance of :10 mm. and having a thickness of :2 mm. was surrounded by a tube 8 of an electrically non-conducting ceramic material impervious to ultraviolet radiation. The wall thickness of the tube was aproximately 0.5 mm. and its length was approximately 100 mm. The ceramic material contained i94% by weight of A1 0 and '-6% by weight of SiO Initially, the lamp had a light output of approximately 80 lumens/w. and an ignition voltage of 250 v. After approximately 5000 operating hours, the ignition voltage had increased to 400 v. and the operating voltage to v.; the light output had decreased to 70 lumens/w.

In comparison, a lamp without the ceramic tube 8 but otherwise of the same construction, after an operating time of approximately 1000 hours the ignition voltage had already increased to approximately 500 v. and the operating voltage to approximately v.; the light output had decreased to 50 lumens/w. Brown crystals were found in the discharge tube 1, while the envelope 7 showed a brown discoloration.

The improvement obtained using the ceramic tube 8 is probably due to the fact that the ultraviolet radiation of the tube 1 can no longer release any photo-electrons on the wire 4. Released electrons can reach the outer side of the discharge tube 1, which apparently results in sodium diffusing through the wall of the discharge tube 1 and hence in the occurence of the said disadvantageous phenomena. The same disadvantageous phenomena occur with the use of other alkali metals, for example lithium.

While the invention has been described in connection with a specific embodiment and applications thereof, other modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention which is defined with greater particularity in the appended claims.

What is claimed is:

1. A high-pressure mercury vapor discharge lamp comprising a discharge tube containing mercury and an alkali iodide, a pair of electrodes at opposite ends of said tube, an outer envelope surrounding and spaced from said tube, at least one conductor extending along the discharge tube and electrically connected tonne of the electrodes, and

an electrically non-conducting screen consisting of a material substantially impervious to ultraviolet radiation surrounding said conductor.

2. A discharge lamp as claimed in claim 1, in which the screen impervious to ultraviolet radiation is a glass tube.

3. A discharge lamp as Claimed in claim 1, in which the screen impervious to ultraviolet radiation is a tube of ceramic material.

4. A discharge lamp as claimed in claim 3, in which the Wall thickness of the tube of ceramic material is at least 0.2 mm., while the material of the ceramic tube contains approximately 94% by weight of A1 0 and approximately 6% by Weight of SiO References Cited UNITED STATES PATENTS 3,424,935 1/1969 Gungle et a1 3l3184 X JAMES W. LAWRENCE, Primary Examiner 10 R. F. HOSSFELD, Assistant Examiner 

